Cytolethal distending toxin (Cdt) is a bacterial toxin with cytotoxic activity. Accordingly, Cdt and portions thereof may be effective for destroying or inhibiting the growth of undesirable cells, such as cancer cells. Cdt and portions thereof may, therefore, be useful for treating or preventing diseases related to the growth of undesirable cells. Nevertheless, there are obstacles to the effective use of Cdt and portions thereof to inhibit the growth of cells. For example, inefficient delivery of Cdt and portions thereof to the interior of the cell may lessen the cytotoxic effects. Accordingly, there is a need for improved Cdt and portions thereof.